1. Field of the Invention
The present invention relates to an image pickup apparatus, and in particular, to an image pickup apparatus including a photographing optical system with a zoom function.
2. Description of the Related Art
Image pickup apparatuses such as electronic cameras, which record images photographed using a CCD (Charge Coupled Device) or a CMOS (Complementary Metal-Oxide Semiconductor) as an image pickup element, are commonly used. Recently, many of the image pickup elements used for such applications have had more than 10 million pixels in order to increase the resolution of still images.
In image pickup apparatuses with such a high resolution image pickup element, the resolution of photographed images is reduced using a cropping method of cutting out and reading only particular pixel areas. This allows image processing to be achieved at a higher speed than reading of all the pixels.
Furthermore, for example, for single lens reflex digital cameras with which still images are conventionally photographed by using an optical viewfinder to observe fields, those which enable observation of the fields in a live-view manner and moving image photographing have started to be commercially available.
On the other hand, image pickup apparatuses are well known which have an electronic zoom function of cutting out a partial area from output image data to generate a quasi-zoom image.
In an image pickup apparatus with such an electronic zoom function, a focus adjustment area is often changed depending on the size of the partial area cut out by electronic zooming.
Furthermore, according to Japanese Patent Application Laid-Open No. 2005-033508, the position of an object is determined based on a selected focus adjustment area, and the partial area position is determined based on the position of the object. Thus, even if the object is not present in the center of the image, the object can be captured in the center of the electronic zoom image.
In the image pickup apparatus with the electronic zoom function as described above, the object is always captured in the center of the image. Thus, even if electronic zooming is performed during live-view display or moving image photographing, the photographing can be achieved without failing to place the object within the partial cutting out area.
However, in image pickup apparatuses using an optical lens with the zoom function as in, for example, the above-described single lens reflex digital camera, zooming by such an optical lens enlarges and reduces a central portion of an image pickup plane. Hence, the object may fail to be placed within the photographing area.
FIG. 8A and FIG. 8B are diagrams illustrating photographed images corresponding to the photographing area of an image pickup element when live view photographing is performed using the zoom function based on the optical lens. FIG. 8A illustrates a photographed image obtained before zooming. FIG. 8B illustrates a photographed image obtained after the zooming.
In FIG. 8A, if the object stands slightly away from the center of the photographing area, when zooming is performed without taking any appropriate measure, the object is displaced out of the displayed live-view image as illustrated in FIG. 8B because only the central area of the photographed region is enlarged.
To prevent the object from being displaced out of the displayed live-view image, the photographer needs to perform framing so as to intentionally maintain a frame image to be photographed. Furthermore, when zooming is to be performed during moving image photographing, if the photographer desires to keep the position of the object in the displayed live-view image unchanged, the framing needs to depend on the zoom amount of the optical lens.
Moreover, if the photographer cannot change a frame image to be photographed as in the case of remotely controlled photographing using a tripod or the like, the zoom function cannot be used for the object unless the object is positioned in the central portion of the image pickup plane, even if a photographing lens with an auto power zoom function is used.